Within the United States, FM audio broadcast channels are broadcast in 200 kHz wide channels in a frequency band from 88 MHz to 108 MHz, and AM audio broadcast channels are broadcast in 10 kHz wide channels in a frequency band from 540 kHz to 1710 kHz. Certain radio broadcasts include digital content along with analog content, for example, as part the broadcast channels used for AM or FM audio broadcasts. Protocols for HD (High Definition) Radio in the United States were developed by iBiquity Digital Corporation. In particular, for HD Radio, digital content is broadcast in upper and lower sidebands related to a center frequency for a primary analog AM or FM broadcast channel.
FIG. 1A (Prior Art) is a signal diagram for an FM audio broadcast channel 100 that includes HD Radio digital content. This HD digital content is transmitted as OFDM (orthogonal frequency division multiplexed) signals within sidebands 104 and 106 on either side of the analog FM signal 102, which is centered on the center frequency (fC) for the FM channel. Sidebands 104 and 106 each include ten frequency partitions. As depicted, digital sideband 104 is located within a lower sideband frequency range (fL1 to fH1), and digital sideband 106 is located within an upper sideband frequency range (fL2 to fH2). The HD Radio spectrum for AM broadcast channels is similarly configured with the HD digital content being in sidebands on either side of the analog AM signal centered within the AM broadcast channel. It is noted that the HD digital content in the upper and lower sidebands 104 and 106 duplicate each and other, and the HD digital content also typically duplicates the analog content within the broadcast channel. The HD digital content can also include content in addition to what is broadcast within the analog content. For example, the HD digital content may include one or more digital-only programs in addition to the digital content that duplicates the content on the analog channel. It is also noted that OFDM signals for HD digital content can be transmitted in an all digital mode such that the analog signal is replaced with the OFDM signals. As such, only HD digital content is transmitted within the channel.
FIG. 1B (Prior Art) is a signal diagram for a pulse shaping function 150 used for shaping OFDM (Orthogonal Frequency Division Multiplex) symbols used for HD Radio broadcasts. OFDM is a modulation technique that encodes digital data on a number of closely spaced orthogonal sub-carrier signals, and OFDM is used for transmitting digital content within the upper and lower sidebands 104 and 106 for the HD Radio broadcast spectrum 100 as shown in FIG. 1A (Prior Art). Each of the ten sub-carriers in the digital sideband is QPSK (quadrature phase shift keying) modulated at a symbol time (T) of 2.9 milliseconds (i.e., T=2.9 ms). Each OFDM symbol includes X+R samples formed by adding a cyclic prefix of R samples (e.g. R=112 samples) to an X-sample OFDM signal (e.g., X=2048 samples). The cyclic prefix is typically formed by prefixing a repetition of the last R samples of the X-sample OFDM signal to form an OFDM symbol having a total of X+R samples (e.g., X+R=2048+112=2160 samples per OFDM symbol). Prior to broadcasting, each OFDM symbol is weighted in the time domain by the pulse shaping function 150, which applies a weight from 0 to 1.0 to each of the 2160 samples within the OFDM symbol. As can be seen in FIG. 1B, this pulse shaping function will cause samples at the beginning and end of OFDM symbol to have reduced weight as compared to the other samples within the OFDM symbol.
To facilitate operation of HD Radio receivers or other digital radio receivers, it is desirable to detect the presence of digital content within the broadcast channel. For example, when an AM or FM broadcast channel is selected for reception in an HD Radio receiver, the HD Radio receiver will typically perform HD demodulation on the received channel and then attempt to detect OFDM digital signals within the demodulated signals by correlating the demodulated received signal with delayed versions of itself. The following equation, for example, can be applied for this delayed-version correlation technique using HD demodulation:c(n)=c(n−1)+z(n)z*(n−X)−z(n−R)z*(n−X−R), where n=0 . . . X+R−1  [EQUATION 1]
For EQUATION 1, z(n) represents a band-limited, complex OFDM signal sampled at 744.1875 ks/s (kilo-samples per second); z*(n−X) represents the complex conjugate of z(n−X); and c(n) represents the element correlation vector of an OFDM symbol. The element correlation vector c(n) has X+R samples (e.g., X+R=2048+112=2160) and is typically averaged over multiple demodulated symbols to reduce the effect of noise. A peak in the averaged element correlation vector c(n) is typically deemed to indicate the presence of an OFDM signal.
The HD demodulation correlation technique described above, however, has significant time and computational requirements that can degrade performance and increase device sizes. For example, reliable detection of OFDM signals in the presence of noise and impairments, such as multipath distortion, requires applying this HD demodulation correlation technique to average over 100 demodulated OFDM symbols. This required averaging for reliability leads to detection times of 300-400 milliseconds or more. Further, this HD demodulation correlation technique requires eight real multiplications and six real additions for each sample at a symbol rate (1/T) of 363.4 Hz. Thus, a total of about 11 million or more arithmetic operations are required per second for this technique, thereby leading to increased computational circuitry and related power requirements. Still further, the amount of memory required for this correlation technique is 4320 (i.e., 2160×2) complex words or 8640 bytes (i.e., 2160×2×2) thereby leading to increased die size for integrated devices that apply this correlation technique.